The present invention relates to a device for measuring the force exerted by a spring of a supporting grid on a nuclear fuel rod passing through the grid.
The fuel assemblies used in nuclear reactors that are cooled and moderated by water include support grids for supporting fuel rods and/or for holding them at the nodes of a regular array, generally a square array. The grid is often constituted by two sets of mutually crossed plates defining rod-receiving cells. Two facing walls in a given cell are organized so that one of them includes a rigid cell-engaging projections and the other includes a spring, often in the form of a hairpin, straddling a plate and intended to force the rod against the projections. The springs may be single, i.e. they may have a single resilient branch acting on one rod; or they may be double, i.e. they may have two branches each acting on a respective rod placed on a respective side of the plate carrying the spring.
It is important for the springs to exert forces on the rods that do not depart too far from a set value. Various devices have already been proposed for measuring the force exerted by a spring that is single or double, for the purpose of verifying that the force exerted by each spring of a grid lies within a predetermined range of values prior to incorporating the grid in an assembly.
For example, document EP-A-0 501 663 describes a device enabling the force exerted by a double spring to be measured; that device includes a gauge designed to be forced into a cell and including a feeler for transmitting the force exerted by a spring. The gauge is fixed to a centering peg which is designed to engage in the cell adjacent to that in which measurement is being performed and to retain the double spring therein. Forced insertion of the gauge and of the centering peg runs the risk of damaging the spring.
Document DE-A-3 242 407 describes another measuring device having a gauge of a diameter that is substantially equal to that of a fuel rod. A bore is provided in the gauge transversely to its axis at a location that faces the thrust point of the spring when the gauge is pushed home. The bore contains a piston whose outside face carries a measuring element. To measure the force of a spring, the gauge is inserted and hydraulic pressure is exerted on the piston to bring it into a determined position such that the width of the gauge level with the spring thrust point is equal to the nominal diameter of a fuel rod. Given that the piston is hardly capable of projecting from the gauge, it is not possible to completely eliminate friction while the gauge is being inserted and removed.